The exceptional mechanical, superior thermal and electrical properties of carbon nanotubes (CNTs) have made them promising for many engineering applications, such as composite reinforcements, scanning probe tips, field emission sources, hydrogen storage systems, super-capacitors, quantum devices, and biosensors. A significant challenge for both fundamental research and practical applications of CNTs is to disperse CNTs into certain media, such as ethanol, water, or polymers. Since CNTs are insoluble and have the propensity to form bundles due to their strong hydrophobicity and van der Waals attractions, a great deal of effort has been invested to develop efficient and low-cost approaches to realize full dispersion of CNTs.
Approaches can be divided into two categories, namely mechanical dispersion and surface modification. Mechanical methods, such as ultrasonication, high shear mixing, and ball milling, have been commonly employed to disperse one kind of material into another, including CNTs. The shear mixing approach uses a shear force produced by air flow in conjunction with a rapidly moving fluid to disperse and align CNTs. The torque produced by the shear force can make CNTs aligned and straightened along the axial direction. Although the shear mixing method is faster in aligning CNTs, only a marginal proportion of CNTs were completely separated. Ultrasonication and ball milling can also be used to disperse CNTs in solutions or polymers. These two methods are easy to operate but usually take long time to disperse CNTs with low-efficiency. During intensive ultrasonication and ball milling, CNT fracture failure often occurs, which destroys the integrity of the dispersed CNTs. Surface modification can be either chemical or physical. Chemical surface modification enables the CNT surface to be functionalized through reactions with atoms or molecules such as fluorine, alkanes, or by ionic modification to improve their chemical compatibility. Wetting or adhesion characteristics of CNTs can be altered through functionalization to reduce their tendency to agglomerate. For example, an amine group or dangling amine moieties can easily form amide bonds by interacting with carboxylic groups located at the ends, sidewalls, and defect sites of the oxidized CNTs and further induce the formation of salt. This method has been proven to be effective in terms of making CNT solution stable and preventing them from aggregating in the solution state. However, such dispersed CNTs tend to clump together after drying. Physical surface modification is the noncovalent stabilization of CNTs by interaction with certain solvents, such as surfactants, polymers, and biomolecules. For instance, sodium dodecylbenzene sulfonate was used as surfactant to assist the dispersion of CNTs. A major drawback of both chemical and physical surface modifications is that impurity is often introduced into the CNT solution, which is difficult to remove in further processes or applications.
Surface passivation has been widely used to treat semiconductor materials to minimize the surface contribution of the semiconductors to the electrical properties of the device. For example, it is well-known that hydrogen can interact with the dangling bonds (DBs) of Si to form Si—H bonds and thus passivate the density surface of Si, which is primarily in the form of DBs and harmful to Si device performance. However, there are no known reports on dispersion of CNTs using hydrogen passivation (HP).